Variable condenser



June 10, 1930: w. DUBILIER v 1,763,554

VARIABLE CONDENSER Filed July 20. 1922 2 Sheets-Sheet 1 IaJ t 6 0" 4"max" I g p I ATTORNEYS June 10, 1930. w. DUBILIER VARIABLE CONDENSERFiled July 20. 1922 Mic A Z4 I] 11 2 Sheets- Sheet 2 MICA I It /Z4 l IMICA 5&9

mvszvmR By 2), aka.

A TTORNEYJ Patented June 1.0, 1930 WILLIAM DUBILIER, OF NEW YORK, N. Y.,

ASSIGNOB TO. DUBILIEB CONDENSER 603- I POR-ATION, A. CORPORATION OFDELAWARE VARIABLE CONDENSED Application filed July 80,

This invention relates to adjustable or variable electrical condensers,and its objects are: first, to effect a reduction in the dimensions ofvariable condensers per unit capac- 5, ity, and second, to provide avariable condenser structure which is economical to manufacture andreliable, in operation.

Variable condensers such as have heretofore been employed extensively inthe radio art consist, generally, of a series of parallel superposedfixed metal plates between which are interleaved and separated therefromby air spaces, a series of rotatable plates by which the capacity of thecondenser ma be varied-usually by turning a knob. on-

densers of the aforementioned type are known in the artas variable aircondensers.

Due to the fact that the dielectric strength of air is relatively low ascompared with :0 some other dielectric substances, it is necessary toprovide quite a considerable space between the fixed and rotatableplates, and, furthermore, a problem which continues to subsist in thesecondensers resides in the 3 matter of adjusting the several rotatableplates with respect to the fixed plates so that the clearancetherebetween will be sutlicient at all points. This difiiculty is onewhich renders it necessary to provide more space :3 between the fixedplates than might otherwise be necessary. As a result of thesecontrolling factors, variable air condensers are necessarily of muchlarger size than would otherwise be necessary for a given capacity, foras is well known, the capacitance between a pair of parallel platesvaries with different dielectrics and inversely with the separation.Variable condensers have, heretofore, been constructed with soliddielectric between the plates for the purpose of overcoming theobjectionable features of variable air condensers-as outlined. above,but, in so far as it has been ascertained these latter have not beenaltogether successful, due, in part, to excessive, friction between therotatable plates'and .the solid dielectric. Hard rubberorsome otherdielectric having a relatively high co-elficient of friction has beenemployed in these condensers The present invention is based largely on1922. Serial No. 576,235.

the discovery that sheet mica, which has by nature an exceedingly smoothsurface, may be successfully utilized to provide a variable condenser inwhich the element of friction is so low as to be negli ible, while atthe same time permitting o a slight pressure being exerted tending tohold the rotatable electrode and the stationary electrodes in closerelation. Furthermore, the dielectric strength of mica is several timesthat of hard rubber and conse uently the thickness of the dielectric maye correspondingly less. This factor is of particular advantage incondensers adapted for use with high potentias. v

The dielectric constant of mica is of the highest order, thus greatlyincreasing the capacitance per unit area as compared with air condensershaving equivalent separations. A still further advantage in the use ofmica is that it may be readily split into extremely thin sheets ofuniform thickness.

Another advantage accruin from the use of mica for thispurpose is t atan almost impalpable dust having extraordinarily good lubricatinqualitiesis formed by abrasion of the sur ace of the mica after thecondenser has been in use for some time, and the presence of this dustgreatly reduces further wear, and tends to exclude air from between thecontacting surfaces j In its preferred form, the invention comprises acasing of insulatin material having an internal cavity or cham er inwhich are mounted one or more flexible fixed electrodes, each comprisinga pair of thin mica discs between which is interposed a thin sheet ofmetal foil such as tin foil. Each pair of mica discs is secured togetherwith a suitable adhesive. There is also provided within the casing oneor more rotatable flexible metal plates which are interleaved with theaforementioned fixed plates and secured to a shaft which extends-throughan aperture in the I casing and is provided at its outer extremity witha suitable knob for turning.

The mica discs may be of any desired thickness but usually may be of theorder of one or two mils. Thus, it will be seen, the fixed electrodesmay be quite thin and correspondingly flexible, which enables the fixedand movable plates to conform to any irregularities in their respectivesurfaces. The rotatable plates are also, preferably, flexible and asthin as mechanical considerations will permit, and may be, in general,of the order of three to six mils in thickness.

On account of the slight separation between the metal foil in the fixedplates, and the rotatable plates, the capacitance per unit area isgreatly increased as compared with that of a variable air condenser, andis still further increased by virtue of the fact that the dielectricconstant of mica is several times that of air.

Another feature of the invention resides in the fact that the adjacentfixed and movable plates'are pressed together with a comparativelyslight but sulficient force to exclude substantially all air frombetween their contacting surfaces, thus further increasing the capacityper unit area.

Still another feature relates to the means employed for retaining thefixed plates with in the casing, and for exerting the slight pressurebetween the fixed and movable plates. This consists of an annular wirespring which seats in a recess provided in the casing, and includes apair of inwardly projecting portions or fingers which also function asbrushes in establishing an electrical connection with the rotatableplate or plates.

A; further feature relates to an arrangement whereby any desired numberof condenser units may be joined together to form a structure of anydesired capacity. ,This feature is of particular advantage toexperimenters and radio amateurs for reasons which are quite obvious. 4r

A still further feature resides in the provision of calibrations applieddirectly to the condenser casing whereby it is adapted for use withoutthe necessity of mounting on a panel.

The invention will be more clearly understood from the followingdetailed description having reference to the accompanying drawing, inwhich Fig. 1 is an enlarged cross-sectional assembly view of a variablecondenser in' accordance with this invention in which there.

is incorporated only one rotatable plate;

Fig. 2 is a plan view of the upper fixed plate of Fig. 1 with a portionof one of the mica discs broken awaythis view being shown at one-halfthe scale of Fig. 1;

Fig. 3 is a plan view of the rotatable metal plate;

Fig. 4 is a plan view of the lower fixed plate of Fig. 1 with a portionof one of the mica discs broken away;

Fig. 5 is a plan view of the upper portion of the condenser casing takenalong the line 55 of Fig. 1;

Fig. 6 is a view similar to Fig. 5 but ineluding the condenser platesand other parts;

Fig. 7 is a cross-sectional view of an al ternative form of variablecondenser having a plurality of rotatable plates;

Fig. 8 is a cross-sectional view illustrating another alternative formof variable condenser which is made up by joining together separatelyoperable units; and

Fig. 9 is a plan view of the structure of Fig. 8. I

The structure shown in Fig. 1 comprises a casing 1 of hard rubber or anyother suitable moulded insulating material, a cover 2 for the casing ofthe same material, a pair of fixed plates 3 and l, a thin flexiblerotatable metallic -electrode 5, a spindle or shaft 6 to whieh'therotatable plate or electrode 5 is secured, a knob 7 for rotating theshaft 6, and a spring wire retainer 8 for holding the fixed electrodesin place and for exerting the pressure between the fixed and rotatableelectrodes.

Each of the fixed plates 3 and 4 consists of a pair of thin flexiblecircular mica discs '3' and 4: secured together with a suitable adhesiveand a plate or thin sheet of metal foil 3 and at interposed between themica discs. These latter are preferably semi-circular in form as shownin Figs. 2 and 4, and are provided with pig-tails 3 and 1" forconnecting with the metal terminal block 9 to which they are soldered.The total thickness of each of the fixed plates ranges from three to sixmills and is preferably not in excess of the latter figure. Terminalblocks 9 and 1,0

are dovetailed in slots in the casing 1 and are provided with screws 11for making outside connections.

The rotatable plate 5 may be of brass and is of approximately three tosix mils in thickness. It is mounted on the shaft 6 and rigidly securedagainst the shoulder 6the end of the shaft 6 being headed up with theplate 5 in place, as is clearly shown in Fig. 1. The spring wireretainer 8 is sprung into a groove 1 in the casing 1 in which positionit bears against the under side of the fixed plate 4 thereby holding thelatter in place. Fixed plate 3 is, preferably, of smaller diameter than4 and a recess of a depth slightly less than the thickness of plates 3and 5 together is provided in the casing in which these plates areseated. The peripheral portion of the fixed plate 4 rests ontheshoulder 1. This provides a convenient arrangement for spacing thefixed plates. The spring wire retainer 8 is soldered to the terminalblock 10. This retainer is provided with a pair of inwardly projectingportions or fingers 8, the ends of which make contact with the rotatableplate 5 thus establishing ficient pressure between their adjacentsurfaces to substantially exclude any air therebetween. casing 1 bymeans of flat head screws 12.

In the alternative structure of Fig. 7 there are four fixedplates 13'andthree rotatable plates 14,'thelatter being secured to the shaft 15 bymeans of screw -16 which isprevented from turning with respect to shaft15, by in 19. The rotatable plates 14 are suita ly spaced by metalwashers 17, and'the fixed plates are spaced by rings 18 of insulatingmaterial. The thickness of washers 17 and rings 18 is preferably sogauged that the fixed and movable plates are in contact and under slightpressure. Otherwise, the structure of gig. 7 is substantially identicalwith that of Figure 8 illustrates a condenser structure comprising twoduplicate units 20 and21, the

former being superimposed upon the latter.

Each of these units has a rotatable plate and two fixed lates of thetype previously described and tlie two units are held together by bolts22 and nuts 23. The shafts 24, to which the rotatable plates 25 aresecured, are each provided with a squared shank 24' which is designed tofit in a socket 24 of the adjoining shaft. These shafts are, therefore,revolubly joined, as will be quite evident; and the rotatable lates areelectrically connected through the s afts. A better connection may,however, be made between the rotatable plates by wiring the terminalblocks 25 together. The fixed plates may be connected by wiring theterminal blocks 26 together. bined in the manner here described toprovide the desired capacity, and it is not essential that the severalunits be identical. Some of the units may be of large capacity andothers of small capacity.

The top of each unit 20, 21, as well as the tops of the casings 1 ofFigs. 1 and 7, may be,

and are preferably, provided with a cali-' brated scale 27, and apointer 28 for registering with the calibrated scale maybe attached tothe knob. This rovision makes it con- Venient to operate t e condenserindependently of its being mounted on a panel.

As will be seen in the construction shown on Figure 8 the casingcomprises sections,

closing it into engagement with the shaft of the adjacent unit. a

. Owing to theinherent resiliency of the fixed and rotatable plates theyconform quite readily to any surface irregularities in each other thusmaintaining a minimum spacing between the sheets of metal foil 3, 4! andthe Cover plate 2 is secured to the p l y Any number of units may becom-- rotatable plates, and at the same time eliminating any inordinateobstruction to the movement of the rotatable platesdne to slightmisalignment and the like.

Fine mica dust may be sprinkled on the friction surfaces of the platesbefore the condenser is assembled to provide initial lubrication, but ifomitted it will soon accumulate from abrasion of the surfaces of themica discs. The mica dust not only functions as a lubricant, but ittends to fillany air spaces between the plates, and since its dielectricconstant is much higher than air, the result is an increase in unitcapacity.

My condenser can be used to give a desired capacity) by adjustment andretain such cay-allowing the parts to remain in therequired position.For example, in Figure 1, the movable plate 5 in combination with thefixed plate 3, or the two fixed plates 3 and 4 together, can be set forany needed capacity by turning the plate 5 till it reaches the pointwhere the wished for capacity is obtained. The pressure between thesurface of the plate 5 and either fixed plate 3 and 4 may, of course,have any value necessaryto retain the plate 5 in its adjustedposition,while permitting it to be moved in contact with the insulationcovering either of the fixed plates.

I claim: 1. In a variable condenser, a thin fixed electrode elementcomprising a sheet of metal foil and a sheet of thinrnica, a thinrotatable electrode element in superposed relation to said sheet ofmica, and means for gently pressing said elements together,said-electrode elements being sufficiently flexible so that theiradjacent surfaces make contact at substantially all' point's,=thepressure between said electrode elements being insufiicient to effectany excessive obstruction to the movement of said rotatable electrodeelement.

in said chamber, said fixed plate comprising a pair of thin dielectricdiscs and a sheet of metallic foil interposed between said discs,,arotatable flexible metallic plate mounted in said chamber in superposedrelation to said fixed plate, and means for rotating saidrotatableplate.

3. In a variable condenser, a casing re fining a ch'amber,.a pluralityof fixed plates" rotatable plates.

4. In a variable condenser, a casing defining a chamber, a pair of fixedplatesmounted.

in said chamber in superposed relation, a rotatable plate interposedbet-ween said fixed plates, a terminal, and an annular retaining a.

spring for said fixed plates, said retaining spring being connected tosaid terminal an ing a chamber, a pair of fixed plates mounted Iselfcontained in said chamber in superposed relation, a rolitatableplate interposed between said fixed plates, and an annular retainingspring for said fixed plates, said retaining spring having a pair ofinwardly extending fingers contacting with said rotatable plate.

6. In a variable condenser, a casing defining achamber, a pair offixedplates mounted in said chamber in superposed relation, a rotatable plateinterposed between said fixed plates, a terminal on said casing, and anannular retaining spring for said fixed plates, said retaining springbeing secured in a groove in said casing and connected to said terminal,and having an inwardly projecting portion, the end of which makescontact with said rotatable thereof.

7. In a variable condenser, a plurality of interchangeable condenserunits each comprising at least one fixed plate and one rotatable plate,and a shaft for each rotatable plate, said shafts being adapted tointerlink whereby the are rotatable jointly, and means for connectingsaid units together in superposed relation, said means comprising acasing in sections, each section enclosing one of said units.

8. In a variable condenser, a casing, conplate near the axis of rotationdenser units within said casing, each unit comprising a fixed plate anda rotatable plate in superposed relation, and a shaft for each rotatableplate, each end portion of said shaft being adapted to engage theopposite end of another similar shaft whereby both shafts may berotatably joined, the casing being in sections each of which enclosesone of said units.

9. A variable condenser unit comprising, a casing,'condenser unitswithin said casing, each including a fixed plate and a rotatable platein superposed relation, and means whereby said rotatable plate isrotatably oined with the rotatable plate of another similar unit whensaid unit is superposed on such similar unit, the casing being insections, each section enclosing one of said units.

10. A variable condenser unit comprising, a casing, condenser unitswithin said casing, each including a fixed plate and a rotatable platein superposed relation, and a shaft for each rotatable plate, said shafthaving a pro-' jecting portion at one end and a recess at the other endportion, said projecting portion being adapted to engage the recess inthe shaft of a similar unit whereby both said shafts are rotatabletogether and non-rotatablewith respect to each other, the casing beingin sections, each of which encloses one of said units.

11. A condenser comprising a fixed electrodeelement, a movable electrodeelement,

. a sheet of mica between said elements and permanently overlyingsubstantially the entire surface of one of said elements, and means formaintaining the other element in sliding contactwith said mica assa-idmovable element is actuated, the mica permitting rubbing engagementof one of said elements with the mica as said condenser is adjusted.

1:2. A condenser comprising a fixed electrode element, a movableelectrode element, a sheet of mica between said elements and permanentlyoverlying substantially the entire surface of the fixed element, andmeans for maintaining the movable element in sliding contact with saidmica as said movable ele ment is actuated, the inica permitting rubbingengagement of one of said elements with the mica as the condenser isadjusted.

13. In a condenser, a fixed electrode element comprising a plate ofconductive material and a sheet of mica thereover, a rotatable electrodeelement in superposed relation to said sheet of mica, and means forpressing said elements together, one of said elements being flexible sothat their adjacent surfaces may makeinsulated contact at substantiallyall points, the pressure between said elements holding them in suchcontact but permitting movement of said rotatable electrode element, toadjust the capacity ofthe condenser.

l-l. In a condenser, a fixed electrode eleinent comprising a plate ofconductive material and a sheet of insulation thereover, a rotatableelectrode element in superposed relation to said sheet of insulation,and means for pressing said elements together, one of said elementsbeing flexible so that the adjacent surfaces of said elements can makeinsulated contact at substantially all points,

the pressure between said elements maintaining such contact whilepermitting movement of said rotatable element, to ad ust the capacity ofthe condenser.

In testimony whereof I afiix my signature.

WILLIAM DUBILIER.

